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Appendix E
Page 137
SCADA-Modbus® System Guidelines
Query
The Modbus ASCII query must take the form shown below that specifies the
starting register and number of registers to be read:
For example, to read the level channel of the 980 Flow Meter, the query must
be as indicated in Table 38.
The master queries the flow meter using a Read Holding Registers request,
which implies a 4XXXX register reference, to slave device address 01.
The message requests data from holding registers 40007–40008 to obtain the
level information, which requires two registers to store the floating point value.
Note that registers are referenced from zero in the data field.
Table 37 SCADA-Modbus Units of Measure Codes
Unit Code Unit Code
ML 1 GPH 26
AF 2 LPS 27
CF 3 LPM 28
GAL 4 LPH 29
L 5 MGD 30
M3 6 PH 31
IN 7 ORP 32
CM 8 PPM 33
FT 9 PPB 34
M 10 MGL 35
CM2 11 PCTSAT 36
FT2 12 MSIEMENS 37
IN2 13 MICROSIEMENS 38
M2 14 GRAMSPERKG 39
AFD 15 PCTPERDEGC 40
CFS 16 DEGREE_C 41
CFM 17 DEGREE_F 42
CFH 18 MILS 43
CFD 19 VOLTS 44
CMS 20 FPS 45
CMM 21 MPS 46
CMH 22 PCT_O2 47
CMD 23 PCT_H2S 48
GPS 24 PCT_LEL 49
GPM 25 VDC 50
Start
‘:’
Slave
Address
Function
(03)
Start
Address
High
Start
Address
Low
No. of Pts.
High
No. of.
Pts. Low
LRC <CR> <LF>